The LTE system is the newest 3GPP mobile communication system in which the air access technology of the 3G is improved and enhanced, namely LTE air interface protocol. As shown in FIG. 1, the system comprises a physical layer (the L1 in FIG. 1), a media access control (MAC) layer, a radio link control (RLC) layer and a packet data convergence protocol (PDCP) layer, which are respectively in the UE and an evolved NodeB (eNB). Herein, the services provided by the RLC entity in the RLC layer mainly comprise: the transmission service of confirmation data (including PDCP PDU submission success instruction), the transmission service of non-confirmed data, in-order submission (excluding the circumstance of lower layer entities reestablishment) and de-duplication (excluding the circumstance of lower layer entities reestablishment); and the PDCP entity in the PDCP layer not only provides the services of header compression, encryption and integrity protection, but also provide necessarily the in-order submission and de-duplication services under the circumstance of lower layer entities reestablishment.
The reestablishment of the RLC entity in the RLC and the MAC entity in the MAC layer is called as lower layer entities reestablishment for short. The reason why the lower layer entities are reestablished includes handover, radio resource control (RRC) connection reestablishment and so on. The eNB transmits the downlink data to the UE, and the L1 layer inside the UE reports the received data layer by layer up to the service layer (application layer). When the RRC instructs the RLC entity to perform the lower layer entities reestablishment, the RLC entity will submit in-order RLC service data unit (SDU) as much as possible to the PDCP entity. After the RLC SDU is subjected to the procedures of compression, integrity protection (for the signaling), encryption and adding with PDCP Head, the data packet obtained is the PDCP PDU (Protocol Data Unit). When the RLC entity is in acknowledgement mode (AM), the format of the PDCP PDU is as shown in FIG. 2, wherein D/C (Data/Control) is a data/control switch; PDCP SN is the sequence number; the PDCP SN (cont.) in the second byte is the PDCP SN which has not been sent out yet in the first byte; R (reserved) is a reserved bit; and the data refers to SDU. Each PDCP PDU submitted by the RLC entity is associated with a certain count value COUNT. The COUNT is divided into two parts: the high position part is called as hyper frame number (HFN) which is maintained at both ends (sending end and receiving end); the low position part is the PDCP SN which refers to the PDCP SN field fixed in the PDCP Header. When a PDCU PDU is received, the value of COUNT associated with the PDU is required to decode and decompress the ultimate SDU from the PDCP PDU. The HFN is exported from the RX_HFN state variable maintained by the UE.
The present protocol maintains the correct HFN of the PDCP PDU through a mechanism based on windows. The window mechanism is as follows: supposing that the range of the PDCP SN is 0≦SN≦Maximum_PDCP_SN, the size of receiving windows Reordering— Window=(Maximum_PDCP_SN+1)/2, for example, as for the PDCP SN of 12 bits, Maximum_PDCP_SN=4095, Reordering_Window=2048. Supposing that the PDCP SN of the PDCP PDU submitted at the latest to the upper layer entity is Last_Submitted_PDCP_RX_SN, the PDCP SN of the PDCP PDU received next will be Next_PDCP_RX_SN. To be short, the lower boundary of the receiving window is Next_PDCP_RX_SN, and the upper boundary generally equals to Next_PDCP_Rx_SN+2048. When the PDCP PDU in the window is received, the HFN of the PDCP PDU is determined according to the position relationship between the Next_PDCP_Rx_SN and the PDCP SN of the received PDCP PDU, and then is decrypted, decompressed and put in the buffer memory; while the PDU beyond the receiving window is discarded.
After the reestablishment of the lower layer entities, the PDCP entity receives the PDCP PDU submitted by the lower layer entities during the reestablishment of the lower layer entities. If the PDCP SN of the PDCP PDU equals to Last_Submitted_PDCP_RX_SN+1, then starting from the PDU, a whole serial of continuous SDUs are submitted to the upper layer entity, and the Last_Submitted_PDCP_RX_SN is refreshed; and if the SN of the PDCP PDU does not equal to Last_Submitted_PDCP_RX_SN+1, the PDCP PDU will be temporally stored in the buffer memory, and when the PDCP PDU with the PDCP SN equaling to Last_Submitted_PDCP_RX_SN+1 is received next time, there is an opportunity for the PDCP PDU stored in the buffer memory to be submitted to the upper layer entity. If after the reestablishment of the lower layer entities the buffer memory still has the PDCP PDU having not been submitted, then there will be an opportunity for the PDCP PDU not submitted yet to be submitted to the upper layer entity after the reestablishment of the lower layer entities.
For example, if Last_Submitted_PDCP_RX_SN equals to 99, the lower boundary of the window is 100, and the upper boundary is 100+2048=2148. If the SN of the received PDCP PDU equal to 100, the SDU in the PDU is reported, and the upper and lower boundaries of the window are modified as [101, 2149]; if the PDCP SN of the received PDCP PDU equals to 102 which is within the boundary range of the window, the PDCP PDU is temporarily stored in the buffer memory, and the PDCP PDU with the PDCP SN of 102 will be reported after the PDCP PDU with the SN equaling to 100 and 101 are received and reported; and if the PDCP SN of the received PDCP PDU equals to 2400 which is beyond the boundary range of the window, the PDCP PDU will be discarded.
However, under the circumstance that when Last_Submitted_PDCP_RX_SN=Maximum_PDCP_SN during the reestablishment of the lower layer entities, all the PDCP PDUs with SN equaling to 0, 1, . . . , Reordering_Window−1 stored in the buffer memory can not be submitted, at the same time, the value of Last_Submitted_PDCP_RX_SN can not be refreshed forwardly, and PDCP PDUs received subsequently in the range of Reordering_Window+1≦SN<2Reordering_Window will be discarded, which results in that the receipt of the downlink data fails.